1. Field of the Invention
The present invention relates to ball screws and steering systems.
2. Description of the Related Art
Japanese Patent Application Publication No. 2004-353835 (JP 2004-353835 A) describes that a ball screw includes a ball nut and a ball screw shaft and functions to convert rotary motion of the ball nut to linear motion of the ball screw shaft. In such a ball screw, a plurality of balls are accommodated in grooves between the outer peripheral surface of the ball screw shaft and the inner peripheral surface of the ball nut, so that the ball screw converts rotary motion to linear motion and vice versa between the ball nut and the ball screw shaft via the plurality of balls and the grooves. For example, the above ball screw is used in a vehicle steering system to convert rotary motion of the ball nut, which is made by an assist actuator, to axial linear motion of a steered shaft (rack shaft).
In addition to ball screws, roller screws such as the one described in Japanese Patent Application Publication No. 2008-39050 (JP 2008-39050 A) are also known in the art. The roller screws use cylindrical rollers as rolling elements instead of the balls. In both the ball screws and the roller screws, the plurality of rolling elements roll in the grooves and thus move together in the grooves when the nut rotates relative to the shaft. It is therefore necessary to return the rolling elements in the grooves from downstream to upstream in the direction in which the rolling elements move so as to recirculate the rolling elements. Structures having a first deflector (recirculating element), a second deflector (recirculating element), and a return pipe, such as the one described in JP 2008-39050 A, are known as an example of structures that return the rolling elements in the grooves from downstream to upstream. The first deflector deflects the rolling elements rolling in the grooves, the return pipe carries the rolling elements from the first deflector to the second deflector, and the second deflector returns the deflected rolling elements to the grooves.
Specifically, the nut is provided with the first and second deflectors extending along the grooves, and the first and second deflectors are connected by the return pipe extending linearly in the axial direction of the nut. In this case, the rolling elements are deflected from the grooves by the first deflector in a downstream part of the grooves. The deflected rolling elements are then carried to the second deflector through the return pipe and are returned from the second deflector to the grooves.
However, if the direction in which the rolling elements flow changes at an acute angle in the connection portion between the first deflector and the return pipe and the connection portion between the return pipe and the second deflector, the rolling elements may not be able to smoothly pass through these connection portions.
The return pipe may be curved in portions connected to the first and second deflectors so that the direction in which the rolling elements flow changes gradually in these portions. However, if these portions of the return pipe are curved, these portions may be deformed, and the inside diameters of these portions, specifically the inside diameters of the return pipe in the direction of the radius of curvature of the curves of these portions, may be reduced. The rolling elements are therefore subjected to increased resistance when passing through these portions, and such increased resistance hinders smooth flow of the rolling elements.